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CdS quantum dots embedded in poly(butylmethacrylate)
Materials Chemistry and Physics 113 (2009) 507–510
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Optical memory based on photo-activated fluorescence of core/shell CdSe/CdS quantum dots embedded in poly(butylmethacrylate) Georgi G. Yordanov, Gospodinka D. Gicheva, Ceco D. Dushkin ∗ Laboratory of Nanoparticle Science and Technology, Faculty of Chemistry, University of Sofia, 1164 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 1 May 2008 Received in revised form 20 July 2008 Accepted 14 September 2008 Keywords: Optical memory Fluorescence Nanocrystals Quantum dots CdSe/CdS Poly(butylmethacrylate) Nanocomposite Trioctylphosphine oxide
a b s t r a c t This work presents the observation of a photo-activated fluorescence from core/shell quantum dots of CdSe/CdS incorporated in a poly(butylmethacrylate) matrix. Upon illumination with UV-light, the intensity of fluorescence from the quantum dots increases as seen by naked eyes at ambient conditions. This allows its utilization in optical memory media based on thin films of CdSe/CdS polymer nanocomposites suitable for practical application. The quantum dots are synthesized by the hotinjection method and embedded in poly(butylmethacrylate) matrix by radical polymerization with 1,1 -azobis-(cyclohexanecarbonitrile). The fluorescence of quantum dots quenched during the polymerization process, but appeared again after illumination of the nanocomposite material with UV-light. The fluorescence properties of quantum dots are governed by the presence of trioctylphosphine oxide in the matrix, which allows control of the optical memory effect. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanocrystals, known as quantum dots (QDs) possess unique optical properties due to the so-called quantum confinement effect [1–3] and hence find many important practical applications [4–6]. That is why, a wide variety of synthesis methods for QDs have been developed. Although the water-based syntheses [7,8] are relatively fast and simple the hot-matrix syntheses in organic solvents produce QDs of higher quality [9–12] as the one utilized by us here. QDs have been incorporated in various polymers [13–16], glasses [17–20] and organic semiconductors [5,21] in order to prepare nanocomposite materials with novel properties. This paper presents the preparation of a nanocomposite material of core/shell CdSe/CdS QDs and polybutylmethacrylate (PBMA) by means of radical polymerization. It was found that this nanocomposite material possesses the so-called photo-activated fluorescence—an interesting phenomenon, which can be utilized in optical memory devices. Similar memory based on simple CdSe QDs deposited as a dry thin film on substrate has been previously described [22]. However, it is demonstrated here that the utilization of CdSe/CdS-PBMA nanocomposite material as optical memory media could be much more efficient by offering higher
∗ Corresponding author. Tel.: +359 2 8161 387; fax: +359 2 962 5438. E-mail address: [email protected]fia.bg (C.D. Dushkin). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.09.040
reproducibility, better mechanical properties, longer durability of information storage and much higher signal-to-noise ratio. The effect of trioctylphosphine oxide (TOPO) on the fluorescence of QDs is studied during the polymerization process by means of controlling the optical memory effect. 2. Experimental section 2.1. Chemicals Cadmium oxide (CdO, 99%), sulfur (S, purum, 99.5%), Rhodamine 6G (for fluorescence) and pulverized selenium (Se, pure, 99.5%) were from Fluka. Tributylphosphine (TBP, 97%) was from Aldrich. Liquid paraffin and stearic acid were from RA.M.Oil SpA (Italy) and Hatkim SA (Turkey), respectively. Chloroform was of analytical reagent grade from Labscan Ltd. (Ireland). Toluene, chloroform, methanol and acetone were purified by distillation. All other chemicals and solvents were used as received, without additional purification. Tributylphosphine sulfide (TBP-S) solution (0.125 M) was prepared by reaction of sulfur (40 mg, 1.25 mmol) and TBP (0.34 ml, 1.35 mmol), and then dilution with liquid paraffin (10 ml). Tributylphosphine selenide (TBP-Se) solution (0.125 M) was prepared by dissolving selenium (100 mg) powder in TBP (0.5 ml) and then diluted with liquid paraffin (9.5 ml). Butylmethacrylate monomer and 1,1 -azobis-(cyclohexanecarbonitrile) were from Fluka. 2.2. Synthesis of quantum dots For a typical preparation of CdSe cores, a mixture of CdO (50 mg, 0.39 mmol), stearic acid (0.600 g, 2.1 mmol) and liquid paraffin (15 ml) were heated up to ∼260 ◦ C. A solution of tributylphosphine selenide, TBP-Se (1.0 ml, 0.125 M), was then fast injected into the reaction solution [12]. The reaction solution was then cooled down by dissolving it in cold toluene (70 ml) 20 s after the injection of TBP-Se. The obtained
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QDs were purified by standard procedures [10]. For the deposition of CdS shells, CdO (50 mg) was dissolved in a mixture of stearic acid (600 mg) and liquid paraffin (5 ml) at 180 ◦ C in open air and then cooled down to 120 ◦ C. CdSe cores, obtained from the synthesis above, were transferred into the mixture. Tributylphosphine sulfide (TBP-S) solution (2 ml; 0.125 M) was injected at 100 ◦ C and the reaction temperature was raised up to 250 ◦ C within 8–10 min. The obtained CdSe/CdS QDs were purified [10] and dissolved in chloroform (10 ml). Before usage, the dispersion was heated to obtain clear dispersion. The obtained core/shell QDs possessed red-shifted absorbance and fluorescence spectra in comparison with the initial CdSe core QDs, which were considered as evidence for the successful growth of CdS shell, as previously described [12]. 2.3. Characterization of QDs The absorbance spectra of quantum dot dispersions in organic solvent were measured by Jenway 6400 spectrophotometer. The fluorescence spectra were measured by Carl-Zeiss monochromator, equipped with a photomultiplying tube (the samples were excited with UV-light of = 370 nm). Quartz cuvettes were utilized in both absorbance and emission measurements. The nanocrystals were imaged by a transmission electron microscope (TEM) JEM-2100F (JEOL) operated at 200 kV of acceleration voltage. The chemical composition of nanoparticles was analyzed by the EDS (energy dispersive spectroscopy) microprobe of the microscope. X-Ray diffraction (XRD) spectra were recorded at room temperature on a powder diffractometer (Siemens D500 with CuK␣ radiation within 2 range 10–80 deg and step 0.05 deg 2 and counting time 2 s/step). 2.4. Preparation and optical properties of QD-poly(butylmethacrylate) nanocomposites The QD-polymer nanocomposites were prepared in the following way. Butylmethacrylate monomer (Fluka) was polymerized by radical polymerization; the monomer was used as received without further purification. 1,1 -Azobis(cyclohexanecarbonitrile) (Fluka, >97%) was used as a radical initiator in concentration 1.8 mg ml−1 . The samples were polymerized at uniform conditions: sealed in Eppendorf tubes and heated for maximum 2 h in a boiling water bath. For the purpose, 0.5 ml of QDs in chloroform (prepared as described above) was dispersed by ultrasonication for 30 min in butylmethacrylate monomer (24 ml) until optically clear dispersion was obtained. A total amount of 44 mg initiator was dissolved in the so obtained dispersion and the resulting dispersion was divided in two equal parts. TOPO (440 mg) was dissolved into the first part to obtain TOPO-coated QDs (denoted as TOPO-QDs), and the second part of the dispersion of stearate-coated QDs (denoted as St-QDs) was left as a control. The obtained dispersions of QDs were sealed into Eppendorf tubes and were put into boiling water to be polymerized. Samples of TOPO-QDs and St-QDs in PBMA (both polymerized for 2 h) were illuminated with UV-light (Hg-lump; maximum emission at 370 nm; light power density at the sample position 2.5 mW cm−2 ) for maximum 120 h at room temperature. The respective control samples were kept on ambient light. Fluorescence spectra of the UV-illuminated and the control samples are measured at regular time intervals. The light radiation of the UV-source was measured with Research Radiometer (Ealing Electro-optics, Inc.). 2.5. Preparation of QD-polymer films for optical memory In order to prepare a material suitable for an optical memory device the QDs were embedded into PBMA films in the presence of TOPO. QDs were embedded in PBMA film (about 1 mm thick) formed by radical polymerization in the presence of TOPO at 100 ◦ C for 2 h. A mask was then put over the QD-PBMA film and illuminated with UV-light for 12 h, a time interval necessary to observe intensive photo-activated fluorescence from the illuminated regions of the film.
3. Results and discussion Core/shell QDs of CdSe/CdS with an average total size of 4.3 nm were used in all the experiments (TEM image of the QDs is given in supplementary material). The QDs could be dispersed well in butylmethacrylate monomer upon ultrasonication and remained luminescent. In all the cases optically clear polymer nanocomposites were formed after 2 h of polymerization, however, the fluorescence of QDs dramatically decreased during the polymerization process, especially if TOPO is added (Fig. 1). The decrease in fluorescence intensity is represented with the relative integrated fluorescence intensity (PL/PL0 ). The integrated fluorescence of each sample (PL) is divided by the integrated fluorescence of the initial sample (before polymerization or before photo-activation, respectively) (PL0 ) (the respective absorbance
Fig. 1. Changes in the fluorescence spectra during butylmethacrylate polymerization in the cases of (a) St-QDs and (b) TOPO-QDs. All fluorescence spectra were measured at constant conditions.
spectra of the samples before and after polymerization are given in supplementary material). We suppose that the fluorescence quenching during polymerization results from the introduction of defects on the nanocrystals surface. Such defects can be centers for non-radiative relaxation of the excited state thus leading to fluorescence quenching. It is interesting to observe that the addition of TOPO facilitates the fluorescence quenching. Most probably, TOPO molecules stabilize (by coordination) Cd(II) ions rather than QDs in the polymerizing medium. It means that TOPO is not a suitable surface capping agent for the investigated conditions, if one desires effective surface protection of QDs. However, utilizing TOPO in this case may be useful to prepare nanocomposite materials, exhibiting optical memory. If illuminated with UV-light, the fluorescence of both St-QDs and TOPO-QDs in PBMA was increasing (Fig. 2). In the case of TOPO-QDs in PBMA, the increase in fluorescence intensity was much more pronounced and accompanied by significant blue shifting of the emission maximum (Fig. 2b). The respective absorbance spectra in this case also become blue-shifted after UV-illumination, which corresponds to decrease of the nanoparticle size (dissolution of nanocrystals). Both, the absorbance and fluorescence spectra show broadening of the QDs size distribution after UV-illumination (the respective exciton absorbance and emission bands are broadened). For the case of St-QDs in PBMA these processes are less pronounced. These results indicate that TOPO facilitates the surface destruction of QDs as well as the defocusing of size distribution upon UV-illumination of the nanocomposite material. The following hypothetical mechanism of photobleaching during polymerization and of photo-activation upon UV-illumination emerges based on the above facts. The initial QDs (before polymerization) are fluorescent, with low concentration of surface defects. During the polymerization process, reactive species (TOPO, radi-
G.G. Yordanov et al. / Materials Chemistry and Physics 113 (2009) 507–510
Fig. 2. Changes in the fluorescence spectra during the photo-activation process for (a) St-QDs and (b) TOPO-QDs in PBMA. The tubes are turned down to demonstrate the solidity of polymer nanocomposites.
cals, etc.) introduce surface defects, which lead to non-radiative recombination of the exciton and thus photobleaching. Bearing in mind the coordinating properties of TOPO, one can expect that it would stabilize Cd(II) ions rather than QDs thus accelerating these processes. The UV-illumination of the nanocomposite material probably causes photo-induced etching of the surface defects. As a result, the concentration of surface defects decreases and the fluorescence re-appears again. The blue shifting of fluorescence spectra supports this statement, because the average QDs size is expected to decrease as a result of the etching process (see Appendix B supplementary material) much more pronounced for TOPO-QDs in PBMA. The effect of photo-activated fluorescence from QD-PBMA nanocomposites is highly reproducible and irreversible (the photoactivated samples do not photobleach for at least several months during the experiment). Photo-activated fluorescence has been previously observed from thin films of TOPO-coated CdSe QDs [22]. Blue shifting in fluorescence was also reported in ref. [22], but these authors proposed another explanation of the photo-activated fluorescence. According to these authors, the photo-activated fluorescence is a result of accumulation of charges (electrons, coming from the photo-generated excitons in the QDs by the UV-light) in the matrix mediated by the TOPO molecules bound to the QDs surfaces. Further research is under way in order to understand the factors that control the optical imaging reported by us here and thus to reveal the underlying mechanism. The relative fluorescence intensity, PL/PL0 , after photoactivation of CdSe QDs in simple dry films has been reported to be less than 10 [22]. In the system presented here (CdSe/CdS QDs in polymer films) this value can be larger than 30 (Fig. 3), which represents about 3-fold higher signal-to-noise ratio of fluorescence image recording. This might be a result of the core/shell type
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Fig. 3. Relative fluorescence intensity (PL/PL0 ) of St-QDs and TOPO-QDs during (a) the polymerization process and (b) the photo-activation process (recording of image). All fluorescence spectra were measured at constant conditions. Using laser blue light of larger intensity may substantially decrease the recording time.
Fig. 4. Operation of QD-PBMA optical memory: (a) final fluorescent image after 12 h of illumination; (b) recording and (c) reading process.
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QDs used in these experiments. Nevertheless, embedding QDs in a polymer film offers much better mechanical stability and better protection of QDs from oxidation, which is necessary for lasting information storage. The fluorescence of non-illuminated TOPOQD-PBMA nanocomposites is almost completely quenched after the polymerization process. This is very important, because it reduces to minimum the background fluorescence of the nanocomposite film, if it is used as optical memory medium. The St-QDs in PBMA are not suitable for optical memory medium, because the background fluorescence is higher and the signal-to-noise ratio is lower in comparison with the TOPO-QD in PBMA. The operation of optical memory based on QD-PBMA nanocomposite is described in Fig. 4. 4. Conclusions This report demonstrates that films of QD-poly-(butylmethacrylate) nanocomposite, prepared from TOPO-coated QDs, could be successfully utilized for irreversible optical recording of fluorescence images. Toward this goal, quantum dots of CdSe/CdS core-shell type were embedded in PBMA by radical polymerization in the presence and absence of trioctylphosphine oxide (TOPO). The fluorescence of QDs was dramatically quenched during polymerization in the presence of TOPO. However, the fluorescence irreversibly appeared again upon UV-illumination of the polymer nanocomposite. Probably, surface defects are introduced in QDs during the polymerization process, which lead to photobleaching. The next photo-activated fluorescence was explained as a result of UVinduced etching of the nanocrystal surface defects, confirmed with the blue shifts in absorbance and fluorescence spectra of TOPOcoated QDs. Further detailed investigation of this phenomenon is required to reveal the intimate mechanism of photo-activated fluorescence in this case. Acknowledgements This research was supported by the Bulgarian Ministry of Education and Science (Project VUH-09/05). The authors are thankful
to COST D43 Action of the EC. The authors also thank Prof. Hideyuki Yoshimura (Department of Physics, Meiji University, Japan) for the Transmission Electron Microscopy (TEM/EDS). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2008.09.040. References [1] A.D. Yoffe, Adv. Phys. 42 (1993) 173–266. [2] S.V. Gaponenko, Optical Properties of Semiconductor Nanocrystals, Cambridge University Press, 2005. [3] A.P. Alivisatos, Science 271 (1996) 933–937. [4] M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013–2016. [5] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (6488) (1994) 354–357. [6] I. Gur, N.A. Fromer, M.L. Geier, A.P. Alivisatos, Science 310 (2005) 462. [7] P. Lianos, J.K. Thomas, Chem. Phys. Lett. 125 (1986) 299. [8] P. Lianos, J.K. Thomas, J. Colloid Interface Sci. 117 (2) (1987) 505–512. [9] Z.A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 183–184. [10] W. Yu, X. Peng, Angew Chem. Int. Ed. 41 (2002) 2368–2371. [11] L. Qu, Z.A. Peng, X. Peng, Nanoletters 1 (2001) 333–337. [12] G. Yordanov, H. Yoshimura, C. Dushkin, Colloid Polym. Sci. 286 (2008) 1097–1102. [13] J. Lee, V.C. Sundar, J.R. Heine, M.G. Bawendi, K.F. Jensen, Adv. Mater. 12 (2000) 1102–1105. [14] M. Tamborra, M. Striccoli, R. Comparelli, M.L. Curri, A. Petrella, A. Agostiano, Nanotechnology 15 (2004) S240–S244. [15] L. Pang, Y. Shen, K. Tetz, Y. Fainman, Opt. Express 13 (2005) 44–49. [16] A.G. Skirtach, C. Déjugnat, D. Braun, A.S. Susha, A.L. Rogach, G.B. Sukhorukov, J. Phys. Chem. C 111 (2007) 555–564. [17] S.T. Selvan, C. Bullen, M. Ashokkumar, P. Mulvaney, Adv. Mater. 13 (2001) 985–988. [18] C. Bullen, P. Mulvaney, C. Sada, M. Ferrari, A. Chiaserac, A. Martucci, J. Mater. Chem. 14 (2004) 1112–1116. [19] T. Mokari, H. Sertchook, A. Aharoni, Y. Ebenstein, D. Avnir, U. Banin, Chem. Mater. 17 (2005) 258–263. [20] M. Correa-Duarte, Y. Kobayashi, R. Caruso, L. Liz-Marzan, J. Nanosci. Nanotechnol. 1 (2001) 95–100. [21] N. Greenham, X. Peng, A.P. Alivisatos, Phys. Rev. B 54 (1996) 17628. [22] S. Maenosono, C. Dushkin, S. Saita, Y. Yamagushi, Jpn. J. Appl. Phys. 39 (2000) 4006–4012.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Optical memory based on photo-activated fluorescence of core/shell CdSe/CdS quantum dots embedded in poly(butylmethacrylate) Georgi G. Yordanov, Gospodinka D. Gicheva, Ceco D. Dushkin ∗ Laboratory of Nanoparticle Science and Technology, Faculty of Chemistry, University of Sofia, 1164 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 1 May 2008 Received in revised form 20 July 2008 Accepted 14 September 2008 Keywords: Optical memory Fluorescence Nanocrystals Quantum dots CdSe/CdS Poly(butylmethacrylate) Nanocomposite Trioctylphosphine oxide
a b s t r a c t This work presents the observation of a photo-activated fluorescence from core/shell quantum dots of CdSe/CdS incorporated in a poly(butylmethacrylate) matrix. Upon illumination with UV-light, the intensity of fluorescence from the quantum dots increases as seen by naked eyes at ambient conditions. This allows its utilization in optical memory media based on thin films of CdSe/CdS polymer nanocomposites suitable for practical application. The quantum dots are synthesized by the hotinjection method and embedded in poly(butylmethacrylate) matrix by radical polymerization with 1,1 -azobis-(cyclohexanecarbonitrile). The fluorescence of quantum dots quenched during the polymerization process, but appeared again after illumination of the nanocomposite material with UV-light. The fluorescence properties of quantum dots are governed by the presence of trioctylphosphine oxide in the matrix, which allows control of the optical memory effect. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanocrystals, known as quantum dots (QDs) possess unique optical properties due to the so-called quantum confinement effect [1–3] and hence find many important practical applications [4–6]. That is why, a wide variety of synthesis methods for QDs have been developed. Although the water-based syntheses [7,8] are relatively fast and simple the hot-matrix syntheses in organic solvents produce QDs of higher quality [9–12] as the one utilized by us here. QDs have been incorporated in various polymers [13–16], glasses [17–20] and organic semiconductors [5,21] in order to prepare nanocomposite materials with novel properties. This paper presents the preparation of a nanocomposite material of core/shell CdSe/CdS QDs and polybutylmethacrylate (PBMA) by means of radical polymerization. It was found that this nanocomposite material possesses the so-called photo-activated fluorescence—an interesting phenomenon, which can be utilized in optical memory devices. Similar memory based on simple CdSe QDs deposited as a dry thin film on substrate has been previously described [22]. However, it is demonstrated here that the utilization of CdSe/CdS-PBMA nanocomposite material as optical memory media could be much more efficient by offering higher
∗ Corresponding author. Tel.: +359 2 8161 387; fax: +359 2 962 5438. E-mail address: [email protected]fia.bg (C.D. Dushkin). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.09.040
reproducibility, better mechanical properties, longer durability of information storage and much higher signal-to-noise ratio. The effect of trioctylphosphine oxide (TOPO) on the fluorescence of QDs is studied during the polymerization process by means of controlling the optical memory effect. 2. Experimental section 2.1. Chemicals Cadmium oxide (CdO, 99%), sulfur (S, purum, 99.5%), Rhodamine 6G (for fluorescence) and pulverized selenium (Se, pure, 99.5%) were from Fluka. Tributylphosphine (TBP, 97%) was from Aldrich. Liquid paraffin and stearic acid were from RA.M.Oil SpA (Italy) and Hatkim SA (Turkey), respectively. Chloroform was of analytical reagent grade from Labscan Ltd. (Ireland). Toluene, chloroform, methanol and acetone were purified by distillation. All other chemicals and solvents were used as received, without additional purification. Tributylphosphine sulfide (TBP-S) solution (0.125 M) was prepared by reaction of sulfur (40 mg, 1.25 mmol) and TBP (0.34 ml, 1.35 mmol), and then dilution with liquid paraffin (10 ml). Tributylphosphine selenide (TBP-Se) solution (0.125 M) was prepared by dissolving selenium (100 mg) powder in TBP (0.5 ml) and then diluted with liquid paraffin (9.5 ml). Butylmethacrylate monomer and 1,1 -azobis-(cyclohexanecarbonitrile) were from Fluka. 2.2. Synthesis of quantum dots For a typical preparation of CdSe cores, a mixture of CdO (50 mg, 0.39 mmol), stearic acid (0.600 g, 2.1 mmol) and liquid paraffin (15 ml) were heated up to ∼260 ◦ C. A solution of tributylphosphine selenide, TBP-Se (1.0 ml, 0.125 M), was then fast injected into the reaction solution [12]. The reaction solution was then cooled down by dissolving it in cold toluene (70 ml) 20 s after the injection of TBP-Se. The obtained
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QDs were purified by standard procedures [10]. For the deposition of CdS shells, CdO (50 mg) was dissolved in a mixture of stearic acid (600 mg) and liquid paraffin (5 ml) at 180 ◦ C in open air and then cooled down to 120 ◦ C. CdSe cores, obtained from the synthesis above, were transferred into the mixture. Tributylphosphine sulfide (TBP-S) solution (2 ml; 0.125 M) was injected at 100 ◦ C and the reaction temperature was raised up to 250 ◦ C within 8–10 min. The obtained CdSe/CdS QDs were purified [10] and dissolved in chloroform (10 ml). Before usage, the dispersion was heated to obtain clear dispersion. The obtained core/shell QDs possessed red-shifted absorbance and fluorescence spectra in comparison with the initial CdSe core QDs, which were considered as evidence for the successful growth of CdS shell, as previously described [12]. 2.3. Characterization of QDs The absorbance spectra of quantum dot dispersions in organic solvent were measured by Jenway 6400 spectrophotometer. The fluorescence spectra were measured by Carl-Zeiss monochromator, equipped with a photomultiplying tube (the samples were excited with UV-light of = 370 nm). Quartz cuvettes were utilized in both absorbance and emission measurements. The nanocrystals were imaged by a transmission electron microscope (TEM) JEM-2100F (JEOL) operated at 200 kV of acceleration voltage. The chemical composition of nanoparticles was analyzed by the EDS (energy dispersive spectroscopy) microprobe of the microscope. X-Ray diffraction (XRD) spectra were recorded at room temperature on a powder diffractometer (Siemens D500 with CuK␣ radiation within 2 range 10–80 deg and step 0.05 deg 2 and counting time 2 s/step). 2.4. Preparation and optical properties of QD-poly(butylmethacrylate) nanocomposites The QD-polymer nanocomposites were prepared in the following way. Butylmethacrylate monomer (Fluka) was polymerized by radical polymerization; the monomer was used as received without further purification. 1,1 -Azobis(cyclohexanecarbonitrile) (Fluka, >97%) was used as a radical initiator in concentration 1.8 mg ml−1 . The samples were polymerized at uniform conditions: sealed in Eppendorf tubes and heated for maximum 2 h in a boiling water bath. For the purpose, 0.5 ml of QDs in chloroform (prepared as described above) was dispersed by ultrasonication for 30 min in butylmethacrylate monomer (24 ml) until optically clear dispersion was obtained. A total amount of 44 mg initiator was dissolved in the so obtained dispersion and the resulting dispersion was divided in two equal parts. TOPO (440 mg) was dissolved into the first part to obtain TOPO-coated QDs (denoted as TOPO-QDs), and the second part of the dispersion of stearate-coated QDs (denoted as St-QDs) was left as a control. The obtained dispersions of QDs were sealed into Eppendorf tubes and were put into boiling water to be polymerized. Samples of TOPO-QDs and St-QDs in PBMA (both polymerized for 2 h) were illuminated with UV-light (Hg-lump; maximum emission at 370 nm; light power density at the sample position 2.5 mW cm−2 ) for maximum 120 h at room temperature. The respective control samples were kept on ambient light. Fluorescence spectra of the UV-illuminated and the control samples are measured at regular time intervals. The light radiation of the UV-source was measured with Research Radiometer (Ealing Electro-optics, Inc.). 2.5. Preparation of QD-polymer films for optical memory In order to prepare a material suitable for an optical memory device the QDs were embedded into PBMA films in the presence of TOPO. QDs were embedded in PBMA film (about 1 mm thick) formed by radical polymerization in the presence of TOPO at 100 ◦ C for 2 h. A mask was then put over the QD-PBMA film and illuminated with UV-light for 12 h, a time interval necessary to observe intensive photo-activated fluorescence from the illuminated regions of the film.
3. Results and discussion Core/shell QDs of CdSe/CdS with an average total size of 4.3 nm were used in all the experiments (TEM image of the QDs is given in supplementary material). The QDs could be dispersed well in butylmethacrylate monomer upon ultrasonication and remained luminescent. In all the cases optically clear polymer nanocomposites were formed after 2 h of polymerization, however, the fluorescence of QDs dramatically decreased during the polymerization process, especially if TOPO is added (Fig. 1). The decrease in fluorescence intensity is represented with the relative integrated fluorescence intensity (PL/PL0 ). The integrated fluorescence of each sample (PL) is divided by the integrated fluorescence of the initial sample (before polymerization or before photo-activation, respectively) (PL0 ) (the respective absorbance
Fig. 1. Changes in the fluorescence spectra during butylmethacrylate polymerization in the cases of (a) St-QDs and (b) TOPO-QDs. All fluorescence spectra were measured at constant conditions.
spectra of the samples before and after polymerization are given in supplementary material). We suppose that the fluorescence quenching during polymerization results from the introduction of defects on the nanocrystals surface. Such defects can be centers for non-radiative relaxation of the excited state thus leading to fluorescence quenching. It is interesting to observe that the addition of TOPO facilitates the fluorescence quenching. Most probably, TOPO molecules stabilize (by coordination) Cd(II) ions rather than QDs in the polymerizing medium. It means that TOPO is not a suitable surface capping agent for the investigated conditions, if one desires effective surface protection of QDs. However, utilizing TOPO in this case may be useful to prepare nanocomposite materials, exhibiting optical memory. If illuminated with UV-light, the fluorescence of both St-QDs and TOPO-QDs in PBMA was increasing (Fig. 2). In the case of TOPO-QDs in PBMA, the increase in fluorescence intensity was much more pronounced and accompanied by significant blue shifting of the emission maximum (Fig. 2b). The respective absorbance spectra in this case also become blue-shifted after UV-illumination, which corresponds to decrease of the nanoparticle size (dissolution of nanocrystals). Both, the absorbance and fluorescence spectra show broadening of the QDs size distribution after UV-illumination (the respective exciton absorbance and emission bands are broadened). For the case of St-QDs in PBMA these processes are less pronounced. These results indicate that TOPO facilitates the surface destruction of QDs as well as the defocusing of size distribution upon UV-illumination of the nanocomposite material. The following hypothetical mechanism of photobleaching during polymerization and of photo-activation upon UV-illumination emerges based on the above facts. The initial QDs (before polymerization) are fluorescent, with low concentration of surface defects. During the polymerization process, reactive species (TOPO, radi-
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Fig. 2. Changes in the fluorescence spectra during the photo-activation process for (a) St-QDs and (b) TOPO-QDs in PBMA. The tubes are turned down to demonstrate the solidity of polymer nanocomposites.
cals, etc.) introduce surface defects, which lead to non-radiative recombination of the exciton and thus photobleaching. Bearing in mind the coordinating properties of TOPO, one can expect that it would stabilize Cd(II) ions rather than QDs thus accelerating these processes. The UV-illumination of the nanocomposite material probably causes photo-induced etching of the surface defects. As a result, the concentration of surface defects decreases and the fluorescence re-appears again. The blue shifting of fluorescence spectra supports this statement, because the average QDs size is expected to decrease as a result of the etching process (see Appendix B supplementary material) much more pronounced for TOPO-QDs in PBMA. The effect of photo-activated fluorescence from QD-PBMA nanocomposites is highly reproducible and irreversible (the photoactivated samples do not photobleach for at least several months during the experiment). Photo-activated fluorescence has been previously observed from thin films of TOPO-coated CdSe QDs [22]. Blue shifting in fluorescence was also reported in ref. [22], but these authors proposed another explanation of the photo-activated fluorescence. According to these authors, the photo-activated fluorescence is a result of accumulation of charges (electrons, coming from the photo-generated excitons in the QDs by the UV-light) in the matrix mediated by the TOPO molecules bound to the QDs surfaces. Further research is under way in order to understand the factors that control the optical imaging reported by us here and thus to reveal the underlying mechanism. The relative fluorescence intensity, PL/PL0 , after photoactivation of CdSe QDs in simple dry films has been reported to be less than 10 [22]. In the system presented here (CdSe/CdS QDs in polymer films) this value can be larger than 30 (Fig. 3), which represents about 3-fold higher signal-to-noise ratio of fluorescence image recording. This might be a result of the core/shell type
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Fig. 3. Relative fluorescence intensity (PL/PL0 ) of St-QDs and TOPO-QDs during (a) the polymerization process and (b) the photo-activation process (recording of image). All fluorescence spectra were measured at constant conditions. Using laser blue light of larger intensity may substantially decrease the recording time.
Fig. 4. Operation of QD-PBMA optical memory: (a) final fluorescent image after 12 h of illumination; (b) recording and (c) reading process.
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QDs used in these experiments. Nevertheless, embedding QDs in a polymer film offers much better mechanical stability and better protection of QDs from oxidation, which is necessary for lasting information storage. The fluorescence of non-illuminated TOPOQD-PBMA nanocomposites is almost completely quenched after the polymerization process. This is very important, because it reduces to minimum the background fluorescence of the nanocomposite film, if it is used as optical memory medium. The St-QDs in PBMA are not suitable for optical memory medium, because the background fluorescence is higher and the signal-to-noise ratio is lower in comparison with the TOPO-QD in PBMA. The operation of optical memory based on QD-PBMA nanocomposite is described in Fig. 4. 4. Conclusions This report demonstrates that films of QD-poly-(butylmethacrylate) nanocomposite, prepared from TOPO-coated QDs, could be successfully utilized for irreversible optical recording of fluorescence images. Toward this goal, quantum dots of CdSe/CdS core-shell type were embedded in PBMA by radical polymerization in the presence and absence of trioctylphosphine oxide (TOPO). The fluorescence of QDs was dramatically quenched during polymerization in the presence of TOPO. However, the fluorescence irreversibly appeared again upon UV-illumination of the polymer nanocomposite. Probably, surface defects are introduced in QDs during the polymerization process, which lead to photobleaching. The next photo-activated fluorescence was explained as a result of UVinduced etching of the nanocrystal surface defects, confirmed with the blue shifts in absorbance and fluorescence spectra of TOPOcoated QDs. Further detailed investigation of this phenomenon is required to reveal the intimate mechanism of photo-activated fluorescence in this case. Acknowledgements This research was supported by the Bulgarian Ministry of Education and Science (Project VUH-09/05). The authors are thankful
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